I see there is one on both the air cooled and the water cooled engines.  The name explains what the purpose of it is, I guess.  But is it adjustable or what role may this play in my lifters not being able to bleed down suddenly.  Is it stuck, or can it be stuck?  And if it is, what disadvantage may it cause?  How important its role is?
Anyone of you gurus know of its problems?
Thanks.  I'm not giving up.
Zoltan